This invention relates to the package or case for transistors and, in particular, to an improved mounting base for transistors dissipating large amounts of power.
In the past, high power semiconductor devices have been characterized by obvious bulk, long thought necessary to remove the heat from the semiconductor chip or die. In general, the concept has been to provide a mass of material in good thermal contact with the die to remove the heat generated during operation of the device. In what is known as the TO-3 type case, the diamond shaped mounting base or header may comprise nickel plated steel or aluminum. Aluminum is used in premium or high power devices due to its superior thermal conductivity.
Aluminum is more costly than steel and is not as strong. The problem with steel is its lower thermal conductivity. In the past, demands for progressively higher power capability have resulted in aluminum being the material chosen for the header. The resulting device is typically a thick aluminum base having a thick copper pedestal to which the semiconductor die is attached, the combination acting as a heat sink for the device.
In view of the foregoing, it is therefore an object of the present invention to provide an improved mounting base for semiconductor devices.
Another object of the present invention is to provide a steel mounting base at least equal in thermal characteristics to aluminum mounting bases.
A further object of the present invention is to provide a mounting base having lower material requirements than those of the prior art.
Another object of the present invention is to provide a mounting base making more efficient use of materials than mounting bases of the prior art.
The foregoing objects are achieved in the present invention wherein a steel mounting base is provided with a central counterbore having a diameter slightly less than the outside dimension of the package. This provides a relatively thin mounting base area in which is welded or brazed a thin copper pedestal. The pedestal has a smaller area than the counterbore and a larger area than the semiconductor chip or die fastened thereon.